There is known a helical gear device comprising a first helical gear disposed rotatably about a first axis; a third helical gear disposed rotatably about a third axis parallel to the first axis; a second helical gear meshing with the first helical gear and the third helical gear and having a center bore; and a support shaft extending through the center bore and disposed coaxially with a second axis parallel to the first axis, so as to support the second helical gear rotatably about the second axis, and wherein a rotary motion is transmitted from the first helical gear to the third helical gear through the second helical gear. JP-2014-13055A discloses an example of such helical gear device, in this helical gear device which is a planetary gear set 46, a second pinion gear 56 corresponds to the first helical gear, and a first pinion gear 52 corresponds to the second helical gear, while a sun gear 48 corresponds to the third helical gear. The second helical gear in the form of the first pinion gear 52 is supported freely rotatably by a support shaft in the form of a first pinion shaft 62 through a needle bearing 66.
However, the helical gear device described above suffers from a risk of concentric misalignment of the second helical gear due to a radial clearance (radial play) of the needle bearing, giving rise to generation of a mutual meshing interference between the second and third, helical gears with an amount of backlash therebetween being zeroed as a result of a movement of the second helical gear toward the third helical gear. The risk of generation of the mutual meshing interference increases with a decrease of a preset amount of the backlash to reduce generation of a mutual tooth butting noise of the second and third helical gears. The generation of the mutual meshing interference of the second helical gear with respect to the third helical gear causes not only considerable deterioration of efficiency of power transmission between the second and third helical gears, but also uneven load application to the second helical gear due to an axial component of a load of meshing with the first helical gear, giving rise to a risk of deterioration of durability of the helical gear device due to wearing of its various parts. Where a thrust washer having a small diameter is disposed axially adjacent to the second helical gear, for instance, the axial component of the load of meshing with the first helical gear is boosted (amplified) according to a difference of the diametric dimension of the thrust washer with respect to the second helical gear, giving rise to a risk of application of an excessively large thrust load to the thrust washer, and consequent early wearing and damaging of the thrust washer.